Although I see nix is required, it would be possible to run the ihp-new program with just nix installed, but without needing to change your PATH permanently. e.g. with a shell.nix file with contents:

  { pkgs ? import <nixpkgs> {} }:

  let
    ihp-new = pkgs.fetchzip {
      url = "https://downloads.digitallyinduced.com/ihp-new.tar.gz";
      sha256 = "0prpw46iyrjwzglq38n4kfn68dv2piw6dn8464ra99yl1dml465m";
    };
  in
  pkgs.mkShell {
    packages = [pkgs.cachix ihp-new];
  }

Running nix-shell with this would download the ihp-new program, and you'd have access to it (and postgres, etc., when you run with a shell with direnv enabled), but outside of the nix-shell, your PATH and other things would be unaffected.